When it comes to "sportized" Enfields, there is sporterized then there is sporterized. Just whacking the military stock off is one thing, whereas removing the rear sight ears, swapping the dog-leg bolt handle for a straight, turned-down handle, taking the step out of the floor plate, fitting a sporter stock, etc., etc.
Pictures would give a better idea of what you consider, "sportized".
^^^ Additionally, which "Enfield" are we talking about - M1917, P14, SMLE/No.1, No.4, or No.5 aka Jungle Carbine?
In my experience, a "lightly" sporterized M1917 will tend to have a slightly higher value than the others, simply because the .30-06 cartridge passes the Wal-Mart test. However, with any sporterized rifle, the extent and quality of the mods is the #1 question.
"...bought this No. 1 MK III a few years ago for $75.00..." Had a commercial stock on it. Not that it matters.
Where you are makes a difference to bubba'd values too. So does having proof of good headspace with a Lee-Enfield. Pattern 14 and 17's are only "Enfields" Stateside. (snicker.) No issues with bad headspace with them. Mind you, if the sight carrier ears are cut off the carrier can be replaced for $6.75. There's no replacing the rear sight wing guards.
Anyway, condition still matters. Easier to sell one with the original barrel length and nothing else cut off.
I have seen military rifles sporterized as nicely as any commercial gun. But resale value is still not high.
On "economy sporters" the key to value is whether you can "re-mil" it without too much trouble.
A $6.75 sight? Great.
Stock? Less so now than in the past.
Barrel? Nope.
Drilled and tapped. No.
P14/1917 ears? Not a chance.
No it's not. I've spent a lot of money on building them in years past and couldn't sell them for what I paid for the barrels alone but they will out shoot anything you can buy out of a rack.
Blindstich, not a driveby, I just tend to observe responses to gather info in my head before replying to tidbits. That and Ive been quite busy lately.
Given the consensus Ill probably just keep it. It did belong to my Dad but he says theres nothing special about, just picked it up along the way some where.